This application is directed to substrate heating apparatus within the preferred embodiment of Chemical Vapor Deposition (CVD) apparatus and more specifically to an improved arrangement for RF heating of the substrate in a deposition reactor.
Chemical Vapor Deposition (CVD) systems are widely used to deposit compositional and compound films in the manufacture of electronic devices, such as integrated circuits formed by the sequential or simultaneous deposition of compounds upon a heated substrate, which is usually in the form of a wafer that is typically mounted on a rotatable or non-rotatable susceptor. The reactants are transported to the surface in the gas phase by typically one or more carrier gases. The elements deposit on the wafer surface, forming the desired compound and the undesirable by-products are pumped away in a gaseous form. The wafers mounted on the susceptor with a heating element mounted below the susceptor to heat the wafers.
Conventionally the heating elements have been radiant (resistive) heating elements made of graphite or tungsten. Deposition of certain materials such as silicon carbide (SiC) semiconductor materials require wafer temperature levels as high as, or above, about 1500° in a hydrogen environment during long deposition runs, e.g., up to 6-8 through a few to several tens of hours. Conventional resistive heating elements, made of graphite or tungsten, cannot stably provide these temperature levels for prolonged periods of time. As a result, RF induction heating has been used when high temperature levels have to be provided in a non-inert environment. In such systems induction coils are operated to produce electric fields in the kilohertz to megahertz range. The RF fields induce eddy currents in the susceptor, which must be of a conducting/metallic material, which in turn heats the wafers.
However conventional induction heating systems are subject to a number of is problems which can lead to failure of the materials to be properly deposited on the substrates. A serious problem is a non uniform temperature profile across the substrates which is caused by factors including some or all of the following: non-uniform induced current heating (i.e. local hot or cold zones), variations in thermal radiation (i.e. sides versus top and bottom surfaces), and irregular configuration of the susceptor among others. These factors can cause an uneven deposition of the materials and/or a failure or certain materials to be deposited at all. Another problem is that conventional induction heating systems require that the susceptor be conductive (metallic) which can result in induced currents in the wafer, causing the mounted substrates to levitate away from the susceptor, or cause the susceptor to warp, again leading to uneven thermal contact to the wafer and thus uneven heating or even a tossing away of the wafer, and contamination directly to the depositing film from the outgassing of the metals/conductors being heated.
The present invention provides an improved RF heating system for a substrate or substrates including a susceptor for supporting the substrate; one or more RF heating coils; and a platen disposed between the RF heating coil and the substrate. The platen is constructed of materials that become heated under RF energy, which will then radiate heat into the susceptor and the substrate. In this way the susceptor need not be constructed of materials that become heated under RF energy thus minimizing levitation. Further, the susceptor may also be made of more inert, higher thermal conductivity, or purer materials, having beneficial properties not realizable from metal. Two such susceptor materials are diamond and alumina for maximum thermal conductivity in an RF headed susceptor-platen system for single or multiple wafers of 2″ to 20″ (or larger) diameter with the susceptor preferably of at least 0.040″ thickness. The platen also provides a uniform temperature profile across the substrates, benefiting from a more diffused heat source. Additionally, the region around the directly RF heated platen can be independently purged to minimize any potential outgassing contaminants.
In a further embodiment the substrate comprises a continuous tape and the substrate supporting device comprises a stationary plate over which the tape passes. An induction heating coil is used to heat a metallic platen which is located under the stationary plate. The heat generated in the platen is radiated to the stationary plate. The stationary plate, as it is heated by radiated heat from the plate, not by eddy currents, can be either insulating or electrically grounded which provides an effective deposition process across a continuous tape, which if heated directly by the RF could not be grounded and thus all metal components in contact with it would become RF conducting.